The present invention relates to a method and apparatus for compensating for thermal distortion for a machine tool.
A machine tool has heat sources in various parts thereof. For example, the heat sources includes a rolling frictional heat of a bearing of a main spindle, a heat generated from a cutting portion and the like. These heats are conducted to respective parts of the machine structure to cause the latter to be deformed. The deformation of the machine structure would largely influence the machining precision.
There have been proposed various methods and apparatus for correcting errors caused by the thermal distortion caused by various causes, by predicting the thermal distortion and feeding a compensation for the errors back to a servo system.
In such a machine tool having a function of compensating, it is important how accurately the thermal distortion generated during the operation of the machine is predicted. Various approaches for this end have been conducted. For example, in one of the approaches, the thermal distortion is predicted from operational conditions such as a rotation speed of the main spindle. In another approach, the thermal distortion is detected by a sensor installed in the machine structure.
The present applicant has proposed a method for compensating a thermal distortion for a machine tool in Japanese Patent Examined Publication No. Hei 6-22779 and Japanese Patent Application Laid-Open No. Hei 3-79256 in which the thermal distortion is calculated from a structure temperature. The calculation according to this method is basically given by the following principle of the following equation (1): EQU .DELTA.L=L.times.(linear expansion coefficient).times.(temperature change)(1)
where .DELTA.L is the thermal distortion of the constituent part of the machine structure, and L is the length of the constituent part.
The machining precision after the compensation according to the conventional technology has a physical limit of about 20 to 30 .mu.m. However, recently, the machine tool users generally require the precision after compensation to be suppressed to 10 .mu.m or less. The reason for this is that a new material such as a ceramic material, a further miniaturized workpiece or the like should be machined with high precision.
Also, in the above-described calculation method, since the length L of the constituent part is expected from the structure of the machine structure and the temperature change is detected at the central position of the length L of the temperature sensor, there is a limit to a mount position of the temperature sensor. Further, in order to predict the thermal distortion with high precision, it is necessary to divide the machine structure into a number of small constituent parts each of which needs a temperature sensor for calculating each temperature change. Also, it is necessary to measure the length L of the constituent part and to confirm linear expansion coefficients of the respective machine constituent parts.
These factors lead to hindrances against the actual mounting work of the thermal distortion compensating apparatus of the machine tool in which the thermal distortion is calculated from the structure temperature.
On the other hand, Japanese Patent Application Laid-Open No. Sho 58-109250 discloses an apparatus for compensating a thermal distortion of a machine tool by using a metal piece having a thermal similarity to the machine tool, regarding the temperature thereof as a temperature representative of the machine tool and controlling a temperature of a cooling jetting air blow. In this case, however, it is necessary to additionally prepare the thermal metal piece having the thermal similarity.
Further, Japanese Patent Application Laid-Open No. Sho 60-9634 discloses a thermal distortion compensating apparatus using a temperature sensor having a thermal time constant which conforms with the characteristics of a Y axis thermal distortion. In this compensating apparatus, however, the detail of the temperature sensor having the thermal time constant which conform to the characteristics of the thermal distortion does not become apparent.
By the way, in case of a machine tool having a plurality of main spindles, each main spindle has a different elongation due to the non-uniformity in prepressure that has been applied to bearings of each main spindle, a difference in conduction of the temperature at a place where each main spindle is mounted and a status of lubrication of the bearings.
For this reason, for example, after a plurality of workpieces have been simultaneously roughly worked and finished by tools mounted on the main spindles, the number of the main spindles to be used in the final finishing work is limited to one and the other main spindles are stopped. As a result, the heat generation of the main spindle is suppressed and only the thermal deformation of the main spindle to be used is thermally compensated to thereby carry out the finishing work. In this case, in order to avoid interferences of the tools carried on the stopping main spindles with the workpieces, it is also necessary to remove the stopping tools from the main spindles in advance. Accordingly, the working efficiency of the finishing work is very low.
Japanese Patent Application Laid-Open No. Hei 5-84628 discloses a thermal distortion compensating apparatus of a machine tool having a plurality of main spindles. However, the thermal distortion compensation through the compensating apparatus has a certain limit. It is difficult to make the machining errors after the compensation approach zero as much as possible.